The glutathione conjugate of prostaglandin A1 (GSH-PGA1) is much more actively reduced (up to 2.4 x 10 to the 4th power times) by PGE2 9-ketoreductase than is PGE2 the presumed natural substrate. The identity of GSH-9OH-PGA1 was established by mass spectrometry. The reductase preferred GSH-PGA1 over L-cysteine-PGA1 and mercaptoethanolamine-PGA1. These results indicate that GSH-PGA1 or A2 occurs naturally and is derived from PGA1 or A2, prostaglandins whose occurrence in man has been doubted. Prostaglandins B1 and B2, E1 and E2 have been resolved by high performance liquid chromatography using a 0.46 x 25 cm Zorbax ODS column and isocratic elution with methanol:0.1% formic acid, 70:30. BIBLIOGRAPHIC REFERENCES: Cagen, L.M., Fales, H.M., and Pisano, J.J.: Formation of Glutathione Conjugates of Prostaglandin A1 in Human Red Blood Cells. J. Biol. Chem. 251: 6550-6554, 1976. Cagen, L.M., Fales, H.M., Bowden, R.E., and Pisano, J.J.: Formation of Glutathione Conjugates of Prostaglandin A1 in Human Erythrocytes. In Silver, M.J., Smith, J.B. and Koesis, J.J. (Eds.): Prostaglandins in Hematology. New York, Spectrum, 1977, pp. 149-157.